The method according to the invention relates in general to transmission in cellular telecommunication systems and in particular to data transfer mode optimization.
In the GSM system, the transmission rate on the TCH channel (TCH/F) used in the transmission between a mobile station MS and a base station BTS is 22.8 kbit/s regardless of the channel coding used. To correct errors, different channel codings are used on the channel that are based on increasing the redundancy of the data transferred: for each information bit it is sent on average more than one bit. The higher the decoding redundancy, the bigger the errors that can be corrected. As the raw transmission speed on the channel is constant, increasing the redundancy decreases the useful transmission rate perceived by the user. Therefore, choosing a channel coding method involves a compromise between transmission reliability and speed.
Coding that offers the fastest transmission rate but the poorest error elimination, namely the 1/2 convolution coding, provides a 12-kbit/s transmission speed for encoded data. In addition to the user's data, the channel also transmits control signals related to the transmission protocol, which leaves a 9.6-kbit/s capacity for the transmission of user data. The more reliable 1/3 convolution coding provides a transmission speed of 4.8 kbit/s, and the 1/6 convolution coding, which is the most effective in eliminating errors, provides a 2.4-kbit/s transmission rate for user data. In the GSM standards these data transfer modes are named TCH/F9.6, TCH/F4.8 and TCH/F2.4. On the half-rate TCH channel (TCH/H) of the GSM system, the 1/2 convolution coding produces a transmission rate of 6 kbit/s for encoded data and thus the user can transmit his data at 4.8 kbit/s, and the 1/3 convolution coding enables a 2.4-kbit/s transmission speed. In the GSM standards these data transfer modes are known as TCH/H4.8 and TCH/H2.4. Data transfer modes that use only channel coding are called transparent modes, abbreviated to T in the GSM recommendations. In these modes the user data transmission rate is predetermined and constant, and the GSM system has no effect on the transmission; only the bit error rate (BER) may vary according to the connection quality.
In addition to channel coding, the GSM system also uses another communication protocol aimed at error elimination, namely, the radio link protocol (RLP). With the RLP protocol in use, the transmitting apparatus packetizes the data into numbered packets, or frames, according to the RLP protocol. If the receiving apparatus does not receive a frame correctly, it can request the transmitting apparatus to re-transmit that frame. If the connection is very noisy, one and the same frame may have to be re-transmitted many times over. This arrangement ensures that the bit error rate (BER) keeps constant, but the transmission rate for user data varies according to the network load and connection quality. Therefore, data transfer modes that use the RLP protocol in addition to channel coding are called non-transparent modes, abbreviated to NT in the GSM recommendations.
The basic structure of the GSM system, known to a person skilled in the art, is described in greater detail in "The GSM System for Mobile Communications" by Michel Mouly and Marie-Bernadette Pautet, ISBN 2-9507190-0-7, Palaiseau 1992, 701 pp. and in the GSM recommendations, for example. These sources also describe in more detail all aforementioned protocols and data transfer modes.
In the current GSM system the user can change the channel coding used by the mobile station. Thus the user is able to maximize the transmission rate in non-transparent data transfer modes. Close to a base station, where connection quality is good, the user should choose a channel coding method that provides the fastest transmission rate (TCH/F9.6). In the fringe areas of the cell, where connection quality is poor and the number of errors is higher, the user should choose channel coding which is better at preventing errors, such as TCH/F4.8, in order to decrease the number of re-transmissions of frames in accordance with the RLP protocol. Currently the GSM system controls the channel coding mode of a mobile station only in fax connections. Therefore, the user has to do the optimization himself, which requires knowledge of the structure of the GSM system. If the user is moving he may have to change the channel coding at short intervals, trying to find the most suitable one. Furthermore, the user has no access to all the information needed for making the optimum choice, such as information about the quality of the connection.
As regards transparent connections, the question of optimizing data transmission arises when the changes in the GSM system and the universal mobile telecommunication system (UMTS), which are now under development, are realized. A new channel coding method enabling a transmission speed of 14.4 kbit/s and a high speed circuit switched data (HSCSD) system are at the moment being developed for the GSM. In the HSCSD system, a user may take into use more than one time slot, ie. more than one TCH channel, in order to increase the transmission speed.
According to plans, the future UMTS system will also have a wider selection of transmission rates than the current GSM system.
In transparent connections the quality of the connection affects only the bit error rate, not the transmission speed. The bit error rate can be dropped only by enhancing channel coding, thereby increasing the amount of encoded data transferred on the transmission path. In the current GSM system this can be done e.g. by changing from the half-rate data transfer mode TCH/H4.8 to the full-rate data transfer mode TCH/F4.8, in which case the transmission rate for user data keeps constant but the channel coding level and thus the amount of raw data transferred via the radio link goes up. Correspondingly, in the HSCSD system the user can take into use more channel resources, or time slots.
So, in the case of transparent connections, data transfer mode optimization has an effect on the quantity of channel resources used. In the HSCSD system, the quantity of channel resources used will affect the user's phone bills, for it is planned that along with the new transmission speeds invoicing will be based, in addition to the current criteria, also on the channel resources used. Thus it will be more expensive to use high transmission rates than low transmission rates that use fewer time slots.
The publication WO 95/28814 describes a method for adapting the data transmission mode in a cellular telephone network as a response to changes in the quality of the radio link between a base station and a mobile station. In the described method, the source coding and channel coding are chosen according to results of two distinct analyses of the transmission quality. The specified method has, however, some drawbacks:
the method does not give a solution to the problem, which arises when every part in the used communication route in the bearer services do not support the chosen transmission mode, PA1 the specified method is limited to use within a single cell, leaving unanswered among others the problems associated with choosing which transmission mode and cell to use, when the current and some neighbouring cells support different transmission mode selections, PA1 the method does not give a solution to the problem, which arises when the transmission mode is changed in the middle of transmission according to the RLP protocol, namely, how to ensure that all RLP frames reach their destination, and PA1 the method does not give a solution to the problems associated with changing data transmission modes in a system, where the use of a more than one channel (timeslot) is available, among others especially the problems associated with maintaining transparent connections in such a system.